(1) Field of the Invention
The present invention relates to chemical reductions catalyzed by dehydrogenase enzymes and more particularly to the implementation of such reductions in the synthesis of methanol.
(2) Description of the Related Art
Methanol is used in a wide range of applications. Among such applications may be noted its use in the production of formaldehyde, in automotive anti-freeze, in a variety of chemical syntheses, as a general solvent, as an aviation fuel (for water injection), as a denaturant for ethyl alcohol, and as a dehydrator for natural gas. Conventional techniques for the production of methanol include high-pressure catalytic synthesis from carbon monoxide and hydrogen, partial oxidation of natural gas hydrocarbons, and purification of pyroligneous acid resulting from destructive distillation of wood.
Various techniques for synthesizing methanol from carbon dioxide are also known. As noted in Enzymatic Conversion of Carbon Dioxide to Methanol: Enhanced Methanol Production in Silica Sol-Gel Matrices, J. Amer. Chem. Soc. 1999, 121, 12192-12193 (published on the World Wide Web on Dec. 9, 1999), partial hydrogenation of carbon dioxide has been carried out by means of heterogeneous catalysis, electro-catalysis, and photocatalysis, with oxide-based catalysts being used predominantly for industrial fixation of carbon dioxide.
Derivation of methanol from carbon dioxide has several obvious advantages. For example, carbon dioxide is plentiful, readily available (indeed, omnipresent), and extremely inexpensive, to say the least. In addition, whereas use of many resources may lead to undesirable depletion of that resource, rising levels of carbon dioxide have been associated with what has been referred to as the xe2x80x9cgreenhousexe2x80x9d effect, which has been theorized to be a contributing factor to global warming. Thus, moderate removal of carbon dioxide from the atmosphere is viewed as beneficial rather than detrimental.
However, conventional methods for synthesizing methanol from carbon dioxide also suffer from certain drawbacks. Such drawbacks include inefficiencies, costs, high energy consumption, and the need for special equipment adapted for high temperature or highly corrosive environments. For example, one common commercial method of methanol synthesis is by reduction of carbon dioxide in the presence of oxide catalysts. However, this synthesis produces partially reduced species as by-products, thereby not only creating impurities but also resulting in limited conversion efficiency. Moreover, the process is carried out at high temperatures, requiring special equipment for accommodating and maintaining such temperatures as well as high energy input.
Various other procedures for reduction of carbon dioxide by enzyme-catalyzed reactions also have been described, but such processes either have not been directed to methanol production or involve various drawbacks. Thus, for example, in CO2 Reduction to Formate by NADH Catalysed by Formate Dehydrogenase from Pseudomonas oxalaticus, Ruschig et al., Eur. J. Biochem. 70, 325-330 (1976), a direct reduction of carbon dioxide by formate dehydrogenase using substrate amounts of NADH is disclosed. The carbon dioxide is reported to have been reduced to formate via carbonate formation in a reaction requiring strict anaerobic conditions to prevent oxygen-induced oxidation of the NADH.
Parkinson and Weaver also describe the production of formate via the formate dehydrogenase catalyzed reduction of carbon dioxide. Photoelectrochemical Pumping of Enzymatic CO2 Reduction, Nature 309, 148 (1984). In their process, Parkinson and Weaver report that a 150 watt tungsten/halogen lamp generated electrons from the semiconductor indium phosphide to reduce methyl viologen (MV2+), which they state mediated the enzyme linked reduction of carbon dioxide to formate. Parkinson and Weaver state that an electrochemical reaction was used to reduce MV2+.
Mandler and Willner discuss relaying photoinduced electrons generated by the (Ru(bpy)3)2+/MV2+ system to an electron transfer molecule such as 2-mercaptoethanol or cystine. Photochemical Fixation of Carbon Dioxide: Enzymic Photosynthesis of Malic, Aspartic, Isocitric, and Formic Acids in Artificial Media, J. Chemical Soc., Perkin Trans., 997 (1988). According to Mandler and Willner, the 2-mercaptoethanol so energized reduced NADP+to generate NADPH, which mediated the enzyme-induced carboxylation of pyruvate to malate. Likewise, Mandler and Willner show that cysteine is capable of donating electrons in the formate dehydrogenase-induced reaction of CO2 to formate. Mandler and Willner note that the formate dehydrogenase activity is problematic because it decays rapidly upon exposure to light, and postulate that since the decarboxylation of formic acid is so energetically favorable, NADH is too weak a reducer to enable efficient production of formate.
Kuwabata et al. describe the sequential reduction of carbon dioxide to methanol by use of formate dehydrogenase and methanol dehydrogenase enzymes, wherein electrons are generated electrochemically and either pyrroloquinolinequinone (PQQ) or MV2+ is used as the electron carrier. Thus, the Kuwabata et al. technique is an electrolytic process that requires everything essential to such processes, including an elecrolytic bath, electrodes, and electrical current input, and also requires use of PPQ or MV2+. Moreover, Kuwabata et al. report that the electrolysis had to be carried out in the dark to maintain the durability of the formate dehydrogenase enzyme.
Accordingly, a new technique for synthesis of methanol, and especially conversion of carbon dioxide to methanol, that alleviates such drawbacks is desired. In particular, a low temperature, highly efficient technique for production of methanol from carbon dioxide is desired.
Briefly, therefore, the present invention is directed to a novel method for conversion of carbon dioxide to methanol. According to the method, a combination of formate dehydrogenase enzymes, formaldehyde dehydrogenase enzymes and either alcohol dehydrogenase enzymes or methanol dehydrogenase enzymes is contacted with the carbon dioxide in the presence of a terminal electron donor to produce the methanol. Preferably, the enzymes are fixed in a microporous matrix such as a sol-gel and the terminal electron donor is a cofactor of the enzymes, such as reduced nicotinamide adenine dinucleotide (which can also donate hydrogen ions to the reductions as well) matrix. In a most preferred embodiment, the invention also contemplates a mechanism for regeneration of the terminal electron donor for reuse.
This synthesis is made up of a series of synthesis steps, from carbon dioxide to formate, from formate to formaldehyde and from formaldehyde to methanol. Thus, the present invention is also directed to each of such steps and sub-combinations of steps, which are assisted by retaining the enzymes in a microporous matrix.
Accordingly, the present invention is also directed to a novel method comprising reduction of formaldehyde to methanol by alcohol dehydrogenase catalysis, to a novel method comprising reduction of formate to formaldehyde by formaldehyde dehydrogenase catalysis, to a novel method comprising reduction of carbon dioxide to formate by formate dehydrogenase catalysis, to a novel method comprising reduction of carbon dioxide to formaldehyde by a combination of formate dehydrogenase catalysis and formaldehyde dehydrogenase catalysis, and to a method comprising reduction of formate to methanol by a combination of formaldehyde dehydrogenase catalysis and alcohol dehydrogenase catalysis.
Among the several advantages found to be achieved by the present invention, therefore, may be noted the provision of a method for synthesis of methanol from carbon dioxide at low temperature; the provision of such method that is energy efficient; the provision of such method that yields high conversion rates; the provision of such method that avoids the need for special equipment adapted to high temperature or highly corrosive environments; and the provision of related methods associated with the intermediary steps of such synthesis.